Method of manufacturing high temperature laminated stator cores

TECHNICAL FIELD

0001The present invention relates generally to electromagnetic devices and, more particularly, to laminated stator cores suitable for usage within electromagnetic devices subject to highly elevated operating temperatures, as well as to methods for manufacturing laminated stator cores.

BACKGROUND

0002Electromagnetic devices, such as motors, sensors, generators, and actuators, commonly include spoked stator cores supporting a number of electromagnetic coils. During operation of the electromagnetic device, the electromagnetic coils magnetically interact with a rotor to provide the desired transducer functionality, such as converting electrical signals to rotor rotation, converting rotor rotation to electrical signals, or converting rotor rotation to power generation. The electromagnetic coils are wound around or inserted over the spokes or posts of the stator core, which is at least partially composed of a magnetically-permeable alloy, such as electrical steel. Cost savings can be realized by producing the stator core as a monolithic structure composed entirely of the magnetically-permeable alloy. Alternatively, the stator core can be produced from laminated stack of magnetically-permeable plates or "laminations," which are separated by intervening dielectric layers (referred to herein as "interlaminate dielectric layers"). While more costly than solid stator cores, laminated stator cores can significantly reduce eddy current losses and, in turn, enhance the efficiency and power density of the electromagnetic device into which the stator core is integrated. As conventionally produced, however, laminated stator cores are generally unsuitable for usage in high temperature applications, such as applications characterized by operating temperatures exceeding about 260 degrees Celsius (°C) or about 500 degrees Fahrenheit (°F).

0003Any one of a number of factors can contribute to the temperature limitations of conventional laminated stator cores. In many cases, laminated stator core temperature limitations are due to the presence of organic dielectric materials within the stator core. As do organic materials, generally, such organic dielectric materials tend to breakdown and decompose at elevated temperatures exceeding the aforementioned threshold. Other factors that can limit the thermal tolerances of laminated stator cores include degradation of the electrically-insulative properties of the interlaminate dielectric due to unfavorable interactions with the laminate alloy under high temperature operating conditions (e.g., mass migration of metal ions from the laminate into the dielectric) and/or the rapid oxidation of the laminates when exposed to air under high temperature conditions. In the majority of applications, such temperature limitations are immaterial as the laminated stator cores are not exposed to such highly elevated operating temperatures. However, in applications wherein the stator cores are subject to such temperatures, the thermal capabilities of conventional laminated stator cores can be undesirably limiting. Such applications can include, but are not limited to, utilization of the laminated stator cores within electromagnetic devices, such as motors, sensors, actuators, generators, or magnetic bearings, deployed within the hot section of a gas turbine engine.

0004Patent document number <patcit id="pcit0001" dnum="US4591529A"><text>US4591529A</text></patcit> describes an electric sheet for manufacturing laminated iron cores for generators, motors, transformers, etc. The sheet is covered on one or both sides with a thin layer consisting of an adhesive applied as a liquid to the sheet and then dried. The sheet has as a filler on one or both sides a reticular skeletal support that is at least partly anchored with the layer of adhesive. The support consists of a fibrous material that accommodates the adhesive.

0005Patent document number <patcit id="pcit0002" dnum="US4103195A"><text>US4103195A</text></patcit> describes bonded laminations forming a stator core, the lamination of the stator core is accomplished by lacking the core with a series of segments or punchings of steel each of which is insulated with a layer of adhesive enamel on both sides. The adhesive enamel is comprised of an epoxy resin and a sufficient amount of a glass fiber spacer filler to substantially provide uniform separation and insulation between the individual segments.

0006Patent document number <patcit id="pcit0003" dnum="US4227108A"><text>US4227108A</text></patcit> describes a laminated iron core for rotary electromachines. The laminated iron core comprises a plurality of metallic plates laminated on each other, a glass layer disposed between opposed side surfaces of each pair of the contiguous metallic plates to hermetically bond the latter together, and an aggregate-containing glass compound layer disposed on at least a portion of the end faces of the laminar metallic plates which is likely to be subject to a thermal/mechanical impact during use in rotary electro-machines.

0007Patent document number <patcit id="pcit0004" dnum="US2003201428A1"><text>US2003/201428A1</text></patcit> describes a dielectric composition for use in formation of a dielectric layer in a plasma display panel. The composition comprising glass powder. The glass powder is powder of glass which contains PbO of 50% or less and CuO as one of essential elements contained in the glass for preventing color change of the dielectric layer from being caused due to reaction with Ag electrodes in the plasma display panel. Ceramics powder can be mixed with the glass powder. The dielectric composition can also be provided in a form of paste, alternatively in a form of a green sheet.

0008A further method of manufacturing a high temperature laminated stator core is also known from <patcit id="pcit0005" dnum="US7788792B2"><text>US7788792B2</text></patcit>.

0009There thus exists an ongoing demand for the provision of laminated stator cores capable of prolonged and reliable operation in highly elevated temperature environments, such as environments characterized by temperatures exceeding about 260°C (∼500°F) and, possibly, approaching or exceeding about 500°C (∼930°F). It is also desirable to provide embodiments of a method for manufacturing such high temperature laminated stator cores. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying Drawings and the foregoing Background.

BRIEF SUMMARY

0010The present invention in its various aspects is as set out in the appended claims.

0011Methods for manufacturing laminated stator cores suitable for usage in high temperature electromagnetic devices are provided. In one embodiment, the method includes obtaining a plurality of coated laminates each comprising a laminate over which a coating precursor layer is formed. The coating precursor layer contains an organic binder and inorganic dielectric particles having a softening point. Subsequently, pre-firing of the coated laminates decomposes the organic binder from the coating precursor layers prior to arranging the plurality of coated laminates in the laminate stack, whereby pre-firing comprises heating the coated laminates to a sintering temperature equal to or greater than the softening point of the inorganic dielectric particles. The plurality of coated laminates are arranged in a laminate stack, which is then fired at temperatures equal to or greater than the softening point of the inorganic dielectric particles. During firing, a compressive force is applied to the laminate stack sufficient to consolidate the inorganic dielectric particles into a plurality of coherent interlaminate dielectric layers electrically insulating and bonding together the plurality of coated laminates as the high temperature laminated stator core.

0012In a further embodiment, the method includes forming oxidation barrier layers on a plurality of laminates and depositing layers of a coating precursor layer (e.g., a glass-containing paste) material over the oxidation barrier layers. The coating precursor material containing glass particles and an organic binder. The organic binder is then thermally decomposed from the layers of coating precursor layer material after deposition thereof. After thermally decomposing the organic binder, the plurality of laminates are laid-up in a laminate stack such that the layers of the coating precursor material are interleaved with the plurality of laminates. The laminate stack is then fired at predetermined processing temperatures exceeding a softening point of the glass particles, while a controlled compressive load is exerted thereon to consolidate the glass particles into a number of interlaminate dielectric layers interleaved with the plurality of laminates.

0013Embodiments of high temperature laminated stator cores are further provided. In one embodiment, the high temperature laminated stator core includes a plurality of laminates each composed of a magnetically-permeable material, such as an iron-cobalt alloy. A plurality of interlaminate dielectric layers is interspersed or interleaved with the plurality of laminates in an alternating arrangement. The plurality of interlaminate dielectric layers electrically insulate and bond together the plurality of laminates. Additionally, the plurality of interlaminate dielectric layers contain consolidated glass particles having a softening point less than the melt point of the magnetically-permeable material and having a coefficient of thermal expansion (CTE) less than the CTE of the magnetically-permeable material.

BRIEF DESCRIPTION OF THE DRAWINGS

0014At least one example of the present invention will hereinafter be described in conjunction with the following figures, wherein like numerals denote like elements, and: <ul id="ul0001" list-style="none"><li><figref idref="f0001">FIG. 1</figref> is flowchart setting-forth an exemplary embodiment for manufacturing laminated stator cores suitable for usage in high temperature operating environments, as illustrated in accordance with an exemplary embodiment of the present invention;</li><li><figref idref="f0002">FIGs. 2</figref> and <figref idref="f0003">3</figref> are isometric and cross-sectional views, respectively, of an exemplary laminate that can be combined with a number of similar laminates to produce the high temperature laminated stator core during performance of the exemplary manufacturing method set-forth in <figref idref="f0001">FIG. 1</figref>; and</li><li><figref idref="f0004 f0005 f0006 f0007">FIGs. 4-7</figref> illustrate a high temperature laminated stator core at various stages of manufacture and produced in accordance with the exemplary manufacturing method set-forth in <figref idref="f0001">FIG. 1</figref>.</li></ul>

0015For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the exemplary and non-limiting embodiments of the invention described in the subsequent Detailed Description. It should further be understood that features or elements appearing in the accompanying figures are not necessarily drawn to scale unless otherwise stated. For example, the dimensions of certain elements or regions in the figures may be exaggerated relative to other elements or regions to improve understanding of embodiments of the invention.

DETAILED DESCRIPTION

0016The following Detailed Description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding Background or the following Detailed Description.Terms such as "over," "under," "on," and the like are utilized to indicate relative position between two structural elements or layers and not necessarily to denote physical contact between structural elements or layers. Thus, a first structure or layer may be described as fabricated "over" or "on" a second structure, layer, or substrate (e.g., laminate) without indicating that the first structure or layer necessarily contacts the second structure, layer, or substrate (e.g., laminate) due to, for example, presence of one or more intervening layers. Furthermore, the term "metal" is utilized herein to encompass both relatively pure metals, as well as alloys. Finally, the term "about," as appearing herein, denotes a disparity of less than 10%.

0017The following describes embodiments of laminated stator cores and methods for manufacturing laminated stator cores having exceptionally high thermal tolerances. Embodiments of the laminated stator cores can provide prolonged and reliable operation in environments characterized by temperatures exceeding about 260°C (∼500°F) and, in certain cases, temperatures approaching or exceeding about 500°C (∼930°F). Laminated stator cores having such high thermal tolerances are referred to herein as "high temperature laminated stator cores," while electromagnetic devices containing high temperature laminated stator cores and also having such high thermal tolerances are likewise referred to as "high temperature electromagnetic devices."

0018By virtue of their exceptionally high thermal tolerances, as well as the high efficiencies and power densities afforded by laminated core designs, embodiments of the laminated stator cores are well-suited for integration into electromagnetic devices utilized within high temperatures applications including, but not limited to, Gas Turbine Engine (GTE) applications, oil drilling applications, and nuclear applications. As a more specific example, embodiments of the laminated stator cores can be integrated into actuators, motors, sensors (e.g., rotary variable differential transformers and motor resolvers), generators, and magnetic bearings deployed within the hot section of a GTE. Additional discussion of electromagnetic devices of this type can be found in <patcit id="pcit0006" dnum="US80147613A" dnum-type="L"><text>U.S. Pat. App. Serial No. 13/801,476</text></patcit>, entitled "GAS TURBINE ENGINE ACTUATION SYSTEMS INCLUDING HIGH TEMPERATURE ACTUATORS AND METHODS FOR THE MANUFACTURE THEREOF," assigned to the assignee of the present Application (Honeywell International, Inc.), and filed with the United States Patent and Trademark Office on March 13, 2013. This patent application is referred to hereafter as the "'476 patent application.

0019<figref idref="f0001">FIG. 1</figref> is a flowchart setting-forth a method <b>10</b> for manufacturing a high temperature laminated stator core, as illustrated in accordance with an exemplary embodiment of the present invention. For purposes of explanation, method <b>10</b> is described below in conjunction with the processing of an exemplary laminate (shown in <figref idref="f0002 f0003 f0004 f0005 f0006 f0007">FIGs. 2-7</figref>) and the production of an exemplary laminated stator core (shown in <figref idref="f0005 f0006 f0007">FIGs. 5-7</figref> at various stages of manufacture). As shown and described below, the illustrated laminate and laminated stator core are provided by way of non-limiting example only. It will be appreciated that laminated stator cores manufactured utilizing method <b>10</b> can be produced to have various other structural forms, as determined by design and application. Further, the fabrication steps described below can be performed in alternative orders, certain steps may be omitted, and additional steps may be performed in alternative embodiments of the manufacture methods. Exemplary method <b>10</b> commences by independently producing, purchasing, or otherwise obtaining a number of coated laminates from which the laminated stator core is subsequently produced (STEP <b>12</b>, <figref idref="f0001">FIG. 1</figref>). The laminates are referred to as "coated" to indicate that each laminate has, at minimum, at least one coating precursor layer formed thereover. The coated laminates may also have other layers formed thereover, such as an oxidation barrier layer of the type described below.

0020<figref idref="f0002">FIG. 2</figref> illustrates an exemplary coated laminate <b>14</b>(<b>a</b>) that may be obtained during STEP <b>12</b> of exemplary method <b>10</b> (<figref idref="f0001">FIG. 1</figref>), along with a number of similar laminates. The coating precursor and oxidation barrier layers formed over laminate <b>14(a)</b> are not shown in <figref idref="f0002">FIG. 2</figref> for clarity, but are shown in subsequent drawing figures and discussed below. As can be seen in <figref idref="f0002">FIG. 2</figref>, laminate <b>14(a)</b> has a relatively thin, platelike shape and includes a generally annular body <b>16(a)</b> from which a number of projections or lobes <b>18(a)</b> extend. A central opening <b>20(a)</b> is formed through body <b>16(a)</b> and defines an inner circumferential surface <b>22(a)</b> of laminate <b>14(a)</b> (also referred to as "inner sidewall <b>22(a)</b>"). Laminate <b>14</b>(<b>a</b>) further includes an outer peripheral surface <b>24(a)</b> (also referred to "outer sidewall <b>24(a)</b>"), an upper principal surface <b>26(a)</b>, and a lower principal surface <b>28(a)</b>. Laminate <b>14(a)</b> can be composed of any suitable magnetically-permeable material and is preferably composed of an alloy containing iron as a primary constituent, such an electrical steels. In one embodiment, laminate <b>14(a)</b> is composed of an alloy containing both iron and cobalt as its primary constituents (referred to here as an "Fe-Co alloy"). The Fe-Co alloy may contain lesser amounts other metallic or non-metallic constituents, such as carbon, silicon, niobium, manganese, and/or vanadium. An example of a commercially-available Fe-Co alloy from which laminate <b>14(a)</b> can be produced is identified as "Hiperco® 50 HS alloy" and marketed by CARPENTER TECHNOLOGY CORPORATION currently headquartered in Reading, Pennsylvania. In one embodiment, laminate <b>14(a)</b> has a thickness between about 100 microns (µm) and about 400 µm. However, in further embodiments, laminate <b>14(a)</b> may be thicker or thinner than the aforementioned range.

0021Laminate <b>14(a)</b> can be produced by cutting the desired laminate shape from a sheet or panel of magnetically-permeable material; the term "cutting," as appearing herein, generally referring all material removal processes including etching, Electrical Discharge Machining (EDM) cutting, and laser cutting. While stamping processes can be employed in certain cases, stamping tends to impart undesired deformation and physical stress to the laminates. Additionally, stamping is prone to the creation of metal shavings or burrs, which can potentially penetrate the interlaminate dielectric layers described below and lessen the effectiveness thereof. For these reasons, laminate <b>14(a)</b> is preferably fabricated from a magnetically-permeable sheet material utilizing a photo-etching process. Advantageously, photo-etching imparts minimal stress on the laminates and creates few, if any, burrs. The etch chemistry employed during the photo etch will vary in conjunction with the composition of the magnetically-permeable sheet material. However, by way of non-limiting example, a ferric chloride (FeCl₃) etch chemistry can be employed when the magnetically-permeable sheet material is composed of an Fe-Co alloy of the type described above. Still further cutting processes that can be utilized to produce the laminates from a selected sheet material include EDM wire and laser cutting processes.

0022The laminates obtained during STEP <b>12</b> of exemplary method <b>10</b> (<figref idref="f0001">FIG. 1</figref>) may each have a coating precursor layer formed thereon and, perhaps, one or more underlying oxidation barrier layers. This may be more fully appreciated by referring to <figref idref="f0003">FIG. 3</figref>, which is a cross-sectional view of laminate <b>14(a)</b> taken along line 3-3 identified <figref idref="f0002">FIG. 2</figref>. As can be seen, two layers have been produced over one or more surfaces of laminate <b>14(a)</b>: (i) an oxidation barrier layer <b>32(a)</b>, and (ii) an overlying coating precursor layer <b>34(a)</b>. These layers are each discussed, in turn, below. The thicknesses of layers <b>32(a)</b> and <b>34(a)</b> are exaggerated in <figref idref="f0003">FIG. 3</figref>, which is not drawn to scale. Laminate <b>14(a)</b>, oxidation barrier layer <b>32(a)</b>, and coating precursor layer <b>34(a)</b> are collectively referred to herein as a "coated laminate" and identified in <figref idref="f0003">FIG. 3</figref> by reference numeral "<b>36(a)</b>." As a point of emphasis, layers <b>32(a)</b> and <b>34(a)</b> can be produced over laminate <b>14(a)</b> after the laminate shape has been cut from a magnetically permeable sheet or panel (referred to here as "laminate singulation"). Alternatively, layers <b>32(a)</b> and <b>34(a)</b> can be formed over laminate <b>14(a)</b> prior to laminate singulation; and, therefore, while laminate <b>14(a)</b> remains interconnected with the other laminates as a relatively large, continuous panel. In this case, laminate <b>14(a)</b> may then be cut from the panel in the above-described manner along with the other, non-illustrated laminates after formation layers <b>32(a)</b> and <b>34(a)</b> over the laminate panel. For example, in an implementation wherein layers <b>32(a)</b> and <b>34(a)</b> are produced on a first (coated) face of the panel, the panel may subsequently be singulated into individual laminates by cutting through the panel (e.g., via photo-etching, laser cutting, or EDM wire cutting) from its opposing, uncoated side.

0023Oxidation barrier layer <b>32(a)</b> can be composed of any material, which decreases the propensity of laminate <b>14</b>(<b>a</b>) to oxidize when exposed to air or another oxidizing ambient at elevated temperatures. As a first example, oxidation barrier layer <b>32(a)</b> can be produced by plating metal (e.g., nickel) over opposing principal <b>26</b>(<b>a</b>) and <b>28</b>(<b>a</b>) of laminate <b>14(a)</b> and, perhaps, over sidewalls surfaces <b>22(a)</b> and <b>24(a)</b>, as well. Alternatively, as a second example, oxidation barrier layer <b>32(a)</b> can be produced by forming a Thermally-Grown Oxide (TGO) layer over laminate <b>14(a)</b>. Both of these approaches are discussed more fully below. In still further embodiments, an oxidation barrier coating system can be formed over the laminates, which may include both a plated metal layer and an underlying TGO layer. If desired, the surfaces of laminate <b>14(a)</b> can be pre-roughed using, for example, a chemical etch, a wet blast, or another roughening technique, prior to application of oxidation barrier layer <b>32(a)</b> to promote adhesion to laminate <b>14(a)</b>. Oxidation barrier layer <b>32(a)</b> may have a thickness between about 0.1 and about 10.0 µm and, preferably, between about 1 and about 3 µm in an embodiment. However, in other embodiments, oxidation barrier layer <b>32(a)</b> may be thicker or thinner than the aforementioned ranges.

0024In a first group of embodiments, nickel or another metal is plated onto laminate <b>14(a)</b> to produce oxidation barrier layer <b>32(a)</b> utilizing either an electrolytic or an electroless plating process. In such embodiments, the preferred plating process may depend upon whether discrete laminates have already been cut from a metal panel at this juncture in the manufacturing process (whether laminate singulation has been performed) or, instead, the laminates remain interconnected as a panel. In implementations wherein the laminates have already undergone singulation and thus exist in their final form (shown in <figref idref="f0002">FIG. 2</figref>), oxidation barrier layer <b>32(a)</b> is preferably produced utilizing electroless plating rather than electroplating, which can produce an uneven build-up of the plated layer at and around the laminate edges (commonly referred to as "dog-boning"). In contrast, when the laminates are processed in panel form prior to singulation, the outer edges of the panel at which dog-boning may occur will typically be removed as scrap during the subsequently-performed singulation process. Thus, in this latter case, electroplating can be utilized to produce an oxidation barrier layer <b>32(a)</b> for cost savings. In either case, heat treatment may be performed after metal plating to diffuse the plated metal layers into the body of laminate <b>14(a)</b> and the other, non-illustrated laminates.

0025In a second group of embodiments, oxidation barrier layer <b>32(a)</b> is formed by the thermal growth of an oxide layer over at least opposing surfaces <b>26(a)</b> and <b>28(a)</b> of laminate <b>14(a)</b>. For example, oxidation barrier layer <b>32(a)</b> can be a TGO layer formed over one or more surfaces of laminate <b>14(a)</b> by heating laminate <b>14(a)</b> to elevated temperatures (e.g., temperatures ranging from about 700 to about 850°C) in an oxidizing atmosphere. The other non-illustrated laminates may also be subject to such a thermal growth process to concurrently form TGO layers thereon. Oxidation barrier layer <b>32(a)</b> can be formed after panel singulation such that laminate <b>14(a)</b> is imparted with in its final form (shown in <figref idref="f0002">FIG. 2</figref>), in which case layer <b>32(a)</b> may extend over sidewall surfaces <b>22(a)</b> and <b>24(b)</b> in addition to principal surfaces <b>26(a)</b> and <b>28(a)</b>. Alternatively, oxidation barrier layer <b>32(a)</b> can be formed over laminate <b>14(a)</b>, while laminate <b>14(a)</b> remains integrally joined to the other laminates as a precut sheet or panel. In this latter case, oxidation barrier layer <b>32(a)</b> will typically not be formed over sidewall surfaces <b>22(a)</b> and <b>24(a)</b>, which have not yet been defined by singulation of the panel. Finally, it will be appreciated that, while such the TGO layers are described herein as grown "over" or "on" the surfaces of laminate <b>14(a)</b> for convenience of description, the TGO will grow both over and into the surfaces of laminate <b>14(a)</b>.

0026A description of the coating precursor layers will now be provided with reference to coating precursor layer <b>34(a)</b> shown in <figref idref="f0003">FIG. 3</figref> for coated laminate <b>36(a)</b> and with the understanding that similar coating precursor layers are produced over the other non-illustrated laminates. As can be seen in <figref idref="f0003">FIG. 3</figref>, coating precursor layer <b>34(a)</b> is applied over upper principal surface <b>26(a)</b> of laminate <b>14(a)</b> and directly onto oxidation barrier layer <b>32(a)</b>. A second coating precursor layer can also be deposited over lower principal surface <b>28(a)</b> of laminate <b>14(a)</b> in further embodiments, but this is not necessary as a second coating precursor will be positioned beneath laminate <b>14(a)</b> during the below-described stacking process (providing that laminate <b>14(a)</b> is not the bottommost laminate included in the laminate stack). Coating precursor layer <b>34(a)</b> contains an inorganic dielectric material in particulate form. The inorganic dielectric particles contained within coating precursor layer <b>34(a)</b> are preferably low melt glass particles; that is, glass particles having a softening point and, perhaps, a melt point less than the melt point of the magnetically-permeable material from which the laminates are produced. For this reason, the following will primarily describe the particles contained within coating precursor layer <b>34(a)</b> and the other non-illustrated coating precursor layers as "glass particles." It is emphasized, however, that other types of inorganic dielectric particles can be contained within the coating precursor material in further embodiments, providing that the inorganic dielectric particles can be consolidated into interlaminate dielectric layers during a consolidative firing process of the type described below in conjunction with STEP <b>54</b> of method <b>10</b> (<figref idref="f0001">FIG. 1</figref>).

0027The inorganic dielectric (e.g., glass) particles contained within coating precursor layer <b>34(a)</b> are ideally formulated for chemical compatibility with the laminate material such that the interlaminate dielectric layers produced by consolidating the inorganic dielectric particles in the below-described manner are resistant to the laminate ion migration. It is also desirable for the coefficient of thermal expansion (CTE) of the inorganic dielectric particles to be generally matched to the CTE of the laminate material, which may be between about 11 and about 20parts per million per degree Celsius (PPM per °C) in an embodiment. In embodiments wherein a disparity exists between the CTEs of the inorganic dielectric particles and the laminate material (as will often be the case), it is preferred the materials are selected such that the inorganic dielectric particle CTE is equal to or less than the CTE of the laminate material, which may range from about 10 PPM per °C to about 20 PPM per °C in an embodiment. In this regard, it is preferred that the CTE of the inorganic dielectric particles is greater than or equal to a minimum CTE threshold and less than the CTE of the laminate material. In one embodiment, and by way of non-limiting example only, the minimum CTE threshold of the inorganic dielectric particles is equal to 9 PPM per °C.

0028Coating precursor materials containing inorganic dielectric (e.g., glass) particles suitable for usage in forming coating precursor layer <b>34(a)</b> can be found in a special class of materials referred to as "ceramic-on-metal dielectrics," which have recently been developed for usage in other applications. It has been found that ceramic-on-metal dielectrics formulated for usage in conjunction with 300 and 400 series stainless steels are well-suited for usage as the coating precursor material (or as a base from which the coating precursor material is formed) when, for example, a Fe-Co alloy is selected as the laminate material. At present, ceramic-on-metal dielectrics suitable for this purpose are available from the following commercial suppliers: ELECTRO-SCIENCE LABORATORIES, INC., headquartered in King of Prussia, Pennsylvania; the FERRO CORPORATION, headquartered in Mayfield Heights, Ohio; and HERAEUS PRECIOUS METALS headquartered in Conshohocken, Pennsylvania. In certain cases, a ceramic-on-metal dielectric material can be acquired and then modified by the addition of one or more refining ingredients to produce a coating precursor material, which is then applied onto laminate <b>14(a)</b>, dried, and prefired to form coating precursor layer <b>34(a)</b>. In this case, additional amounts of a solvent or liquid carrier can be added to optimize the coating precursor material for wet state application onto the laminates (described below). Additionally or alternatively, an acrylic resin or other material can be added to increase the strength of the coating precursor material when dried and prefired.

0029Within continued reference to <figref idref="f0001 f0002 f0003">FIGs. 1-3</figref>, coating precursor layer <b>34(a)</b> can be applied to laminate <b>14(a)</b> utilizing either a wet state application technique or a dry state application technique. When a wet state application technique is used, a flowable or wet state coating precursor material is first obtained by, for example, independent production or purchase from a third party supplier. The wet state coating precursor material may contain inorganic dielectric particles dispersed within an organic binder, such as ethyl cellulose or an acrylic. Additionally, the coating precursor material may contain a solvent or liquid carrier transforming the precursor material to a wet or flowable state. Suitable solvents or liquid carriers include high molecular weight alcohols resistant to evaporation at room temperature, such as alpha-terpineol or TEXINOL®. The volume of solvent or liquid carrier contained within the coating precursor material can be adjusted to tailor of the viscosity of the precursor material to the selected wet state application technique. For example, in embodiments wherein the precursor material is applied by screen printing or doctor blading, the coating precursor material may contain sufficient liquid to create a paste or slurry.

0030Screen printing is a preferred wet state application technique in view of its ability to provide excellent thickness uniformity and minimize waste. In one embodiment, a glass-containing paste is applied to laminate <b>14(a)</b> at a predetermined thickness (e.g., between 10 and 20 µm), which may be approximately twice the final desired thickness of the interlaminate dielectric layers produced from the coating precursor layers, as described below. The thickness of the paste layer can be minimized by utilizing an ultrafine commercial screen and/or by reducing the weight percentage of solids in the paste by, for example, increasing the binder and solvent content. Additionally, calendaring can be employed to reduce the thickness of the paste layer. As a still further possibility, paste layer can be printed in a pattern providing less than 100% surface area coverage providing that non-covered areas are small enough the inorganic dielectric (e.g., glass) particles would flow over entire substrate when wet, fired, or pressed, as described below. The foregoing notwithstanding, wet state application techniques other than screen printing can also be employed to apply a coating precursor layer <b>34(a)</b> to laminate <b>14(a)</b>. Such alternative wet state application techniques include spraying, dipping, and doctor blade application. In the case of spraying and dipping, the coating precursor material may be diluted to a paint-like consistency prior to application onto laminate <b>14(a)</b> and the other non-illustrated laminates. After application of the wet state coating material, a drying process can be carried-out to remove excess liquid from the coating material. In one embodiment, drying is performed at room temperature or at an elevated temperature utilizing an oven or heat gun. Coating precursor layer <b>34(a)</b> results.

0031In further embodiments, a dry state application technique can be employed to apply coating precursor layer <b>34(a)</b> to laminate <b>14(a)</b>. For example, a transfer process or "green tape lamination" process can be utilized wherein coating precursor layer <b>34(a)</b> is first deposited (e.g., screen printed or doctor bladed) and dried onto a temporary substrate or carrier, such as a tape backing (e.g., a strip of Mylar®). In this case, the binder content of the coating precursor material may be increased to, for example, about 8-10 weight percent (wt%) for additional strength. The coating precursor layer <b>34(a)</b> and the tape backing may be positioned over laminate <b>14(a)</b>, and inverted to place layer <b>34(a)</b> in contact with oxidation barrier layer <b>32(a)</b>. Heat and possibly pressure is then applied to adhere oxidation barrier layer <b>32(a)</b> to oxidation barrier layer <b>32(a)</b> and allow removal of the tape backing by, for example, physically peeling the tape away. The structure shown in <figref idref="f0003">FIG. 3</figref> results.

0032Coated laminate <b>36(a)</b> and the other non-illustrated coated laminates are subject to a pre-firing process in method <b>10</b> (<figref idref="f0001">FIG. 1</figref>). When performed, pre-firing enables organic materials contained within coating precursor layer <b>34(a)</b> to be decomposed or burned-out prior to laminate stacking. In so doing, pre-firing can advantageously shorten the manufacturing process by avoiding the need to perform such a burnout step during the below-described consolidative firing process when the coating precursor layer <b>34(a)</b> and the other coating precursor layers are largely shielded from the ingress of oxygen. Pre-firing may entail heating the coated laminates to an elevated temperatures at which the organic binder (and any other organic materials) in the coating precursor materials decomposes, while exposing the coated laminates to air or another oxygen-containing environment. Pre-firing can typically be performed in a relatively short period of time on the order of, for example, 30 to 60 minutes. In one implementation, the coated laminates are transported through a belt furnace, which heats the laminates to a predetermined maximum temperature for a time period sufficient to decompose substantially all organic material from the coating precursor layers; the term "substantially all" denoting that at least 99 wt% of the organic material is removed from the coating precursor materials. In certain embodiments, the pre-firing step will be performed at highly elevated temperatures (e.g., from about 700 to about 850°C) sufficient to glaze, sinter, or slightly melt the inorganic dielectric particles to help strengthen the post-fired coating precursor layers, which may otherwise be weakened when the organic binder is decomposed therefrom. Such highly elevated temperatures causing sintering of the inorganic dielectric (e.g., glass) particles are referred to herein as "sintering temperatures." Coating precursor layer <b>34(a)</b> is still considered to contain inorganic dielectric particles even when the particles are partially merged or sintered together as a result of such a pre-firing process. Advantageously, pre-firing the coated laminates at such temperatures may also serve to heat treat laminate <b>14(a)</b> and the other non-illustrated laminates. Alternatively, the metal laminates can be heat treated utilizing an independent heat treatment step or, instead, as part of the below-described consolidative firing process.

0033Advancing to STEP <b>40</b> of exemplary method <b>10</b> (<figref idref="f0001">FIG. 1</figref>), coated laminate <b>36(a)</b> is next arranged in a laminate stack with a number of other laminates. During stacking, the coated laminates are laid-up in a vertically overlapping relationship such that the laminates are interspersed with or interleaved with the coating precursor layers. A fixture including locating pins or other register features is advantageously utilized during STEP <b>40</b> to ensure proper vertical alignment of the coated laminates. In this regard, <figref idref="f0004">FIG. 4</figref> generically illustrates an exemplary fixture <b>44</b> including a base plate <b>46</b> and a plurality locating pins <b>48</b> (only one of which can be seen) suitable for usage during laminate stacking. In this example, coated laminate <b>36(a)</b> is first loaded onto fixture <b>44</b> and rotated in a first rotational direction until one or more of lobes <b>18(a)</b> contact locating pins <b>48</b>. As indicated by arrow <b>50</b>, a second coated laminate <b>36(b)</b> is then loaded onto fixture <b>44</b>, placed in contact with coating precursor layer <b>34(a)</b> of coated laminate <b>36(a)</b>, and rotated into contact with pins <b>48</b> to ensure proper angular alignment with coated laminate <b>36(a)</b>. As indicated in <figref idref="f0004">FIG. 4</figref> by symbol <b>52</b>, this process is repeated until a desired number of laminates (e.g., a few dozen to several hundred laminates) has been loaded onto fixture <b>44</b>. The laminates loaded onto fixture <b>44</b> are collectively referred to herein as "laminates <b>14</b>," and the reference numbers used to denote the feature of coated laminate <b>36(a)</b> are repeated for each of the illustrated laminates shown in <figref idref="f0004">FIGs. 4</figref>, <figref idref="f0005">5</figref>, and <figref idref="f0007">7</figref> with appropriate changes to the parenthetical following each reference numeral.

0034After stacking laminates <b>14</b> into the laminate stack, the laminate stack undergoes a consolidative firing process (STEP <b>54</b>, <figref idref="f0001">FIG. 1</figref>). During this process, the laminate stack is subject to a compressive loads and elevated temperatures sufficient to consolidate the inorganic dielectric (e.g., glass) particles contained within coating precursor layers <b>38</b> into coherent interlaminate dielectric layers, which are interleaved or interspersed with laminates <b>14</b> in an alternating arrangement. For example, as generically illustrated in <figref idref="f0005">FIG. 5</figref> wherein the laminate stack is identified by reference numeral "<b>58</b>" and only partially shown as indicated by symbol <b>52</b>, the non-consolidated laminate stack <b>58</b> can be enclosed in a hinged furnace jacket <b>56</b>. Although not shown in <figref idref="f0005">FIG. 5</figref> for clarity, an opening may be provided in the upper and/or lower ends of furnace jacket <b>56</b> to the ram portion of a hydraulic press to extend into jacket <b>56</b> and exert a controlled compressive load on laminate stack <b>58</b> (indicated in <figref idref="f0005">FIG. 5</figref> by arrow <b>62</b>), while stack <b>58</b> is heated to elevated temperatures indicated by heat line <b>60</b>) in accordance with a predetermined heating schedule, as described below.

0035In other embodiments, a different means can be utilized to exert a convergent force or compressive preload on the laminate stack during firing; e.g., a bulk weight may be placed on top of the laminate stack during firing. In general, however, it is preferred that the compressive load is applied by an adjustable mechanism, such as a hydraulic press, such that the load can be varied during the consolidative firing process. For example, in one embodiment wherein binder remains in coating precursor layers <b>38</b> at the time of consolidative firing, a relatively light compressive force may initially be applied until the binder softens to a plastic flow state. Afterwards, coating precursor layers <b>38</b> may be leveled by increasing the compressive load. The compressive load may then be reduced during organic burnout (if not previously performed), and then again increased to remove voiding during consolidation of precursor layers <b>38</b> into coherent dielectric layers. Finally, the compressive load can be reduced to a zero value during the cool down cycle. If desired, a number of laminate stacks can be placed in series or in parallel and simultaneously subjected to the consolidative firing process for increased efficiency.

0036In a comparative embodiment, if a pre-firing step was not previously performed such that an organic binder or other organic material is still present in coating precursor layers <b>38</b> (that is, layers <b>38</b> remain in a "green state"), an initial burnout phase can be carried-out during STEP <b>54</b> of exemplary method <b>10</b> (<figref idref="f0001">FIG. 1</figref>). During the initial burnout phase, laminate stack <b>58</b> may be exposed to a first predetermined temperature threshold for a sufficient period of time to decompose the organic material from coating precursor layers <b>38</b>. The first predetermined temperature threshold may range from about 400 to about 500°C in an embodiment. During this phase of the process, a relatively light convergent force can be applied to laminate stack <b>58</b> to maintain the relative positioning of laminates <b>14</b>, while still permitting the ingress of oxygen to promote organic material burnout. In a preferred embodiment, such a burnout phase is performed under process conditions sufficient to remove substantially all organic material from coating precursor layers <b>38</b> such that the interlaminate dielectric layers described below are substantially devoid of the organic binder or any other organic material; that is, contain less than 0.1 wt% organic material. Since pre-firing was performed during STEP <b>40</b> and the organic material was removed from the coating precursor material, the above-described initial burnout phase need not be performed.

0037After the initial burnout phase, laminate stack <b>58</b> may be fired to a second predetermined temperature threshold exceeding the first temperature threshold. The second temperature threshold may be equivalent to or greater than the softening point the inorganic dielectric (e.g., glass) particles contained within coating precursor layers <b>38</b> and less than the melt point of the laminate material. In certain cases, the second temperature may be greater than the melt point of the inorganic dielectric particles, which may be, for example, approximately 100°C greater than the softening point of the particles. After the second temperature threshold is reached, the compressive load exerted on laminate stack <b>58</b> may be increased to a maximum value. This causes the inorganic dielectric particles contained within coating precursor layers <b>38</b> to flow into the voids between neighboring laminates <b>14</b>, merge, and ultimately form a number of coherent interlaminate dielectric layers between laminates <b>14</b> (identified by reference numeral "<b>72</b>" in <figref idref="f0007">FIG. 7</figref>). The term "coherent" is thus utilized to indicate that the interlaminate dielectric layers <b>72</b> produced pursuant to consolidative firing are densified (less porous) as compared to the coating precursor layers and are substantially void free. Interlaminate dielectric layers <b>72</b> are interspersed or interleaved with laminates <b>14</b> in a vertically alternating relationship. Interlaminate dielectric layers <b>72</b> thus provide both electrical insulation between neighboring laminates <b>14</b> included within the laminate stack and bond the neighboring laminates together. For this reason, the post-firing or consolidated laminate stack may be referred to as a "laminated stator core" hereafter. Additional firing cycles may be performed, as needed. Furthermore, if the laminates have not yet been subjected to a metal heat treatment step, the consolidative firing process may also be controlled to heat treat the metal laminates as part of the consolidative firing process.

0038By way of non-limiting example, the final thickness of the interlaminate dielectric layers may range from about 5 to about 10µm after consolidative firing. The interlaminate dielectric layer thickness will be less than that of the coating precursor layers, which may have an initial thickness between about 10 and about 20µm when applied utilizing a wet state application technique of the type described above. In many cases, the compressive load and temperatures applied during the consolidative firing process can be controlled to prevent laminate contact and impart the resulting interlaminate dielectric layers with the desired final thickness. However, when this is not the case or when it is desired to ensure that a minimum gap between the laminates is maintained, inorganic standoff particles can be added to the materials from which coating precursor layers <b>34</b> (<figref idref="f0004">FIG. 4</figref>) are formed. The inorganic standoff particles can be, for example, presorted, spherical particles having a softening point greater than the softening point and possibly greater than the melt point of the inorganic dielectric (e.g., glass) particles contained in the coating precursor material. Suitable materials include high melt glasses and ceramics, such as alumina. The inorganic standoff particles can be presorted utilizing, for example, cyclonic separation to limit the spheres to a tightly defined diameter range corresponding to the desired vertical standoff (as previously determined). Stated differently, presorted inorganic dielectric spheres having a maximum diameter substantially equivalent to the desired vertical standoff can be embedded within the coating precursor layers by, for example, mixing the spheres into the coating precursor material prior to application onto laminates <b>14</b>. During the consolidative firing process, the processing temperatures are held below the softening point of the inorganic standoff particles to ensure the standoff particles maintain their rigidity and thus provide a physical spacer setting the vertical standoff between the laminates and defining the thickness of the resulting interlaminate dielectric layers <b>72</b>. As a somewhat arbitrary example, in an embodiment wherein the desired minimum vertical standoff between neighboring laminates is 8 µm, presorted inorganic spheres having a maximum diameter of about 8 µm (and a minimum diameter somewhere below 8 µm) can be utilized as the inorganic standoff particles. In other embodiments, the coating precursor layers may lack such inorganic standoff particles.

0039Continuing to STEP <b>64</b> of exemplary method <b>10</b> (<figref idref="f0001">FIG. 1</figref>), the laminated stator core is next allowed to cool to, for example, room temperature after consolidative firing. The laminated stator core is then removed from hinged furnace <b>56</b> (<figref idref="f0005">FIG. 5</figref>). The resultant structure is shown <figref idref="f0006">FIG. 6</figref> wherein the laminated stator core is identified by reference numeral "<b>66.</b>" As can be seen in <figref idref="f0006">FIG. 6</figref>, laminated stator core <b>66</b> includes a generally tubular body <b>67</b>, a central opening or longitudinal channel <b>69</b> extending through body <b>67</b>, and a number of stator posts <b>68</b> extending radially outward from body <b>67</b>. Stator posts <b>68</b> are produced from the overlapping or aligning lobes of the individual laminates included within stator core <b>66</b>. Additional processes can be performed to complete fabrication of the laminated stator core (STEP <b>70</b>, <figref idref="f0001">FIG. 1</figref>). For example, one or more sidewall surfaces of the laminated stator core can be machined to final dimensions and then coated with an oxidation barrier, dielectric coating. This may be more fully appreciated by referring to <figref idref="f0007">FIG. 7</figref>, which illustrates a small number of the interlaminate dielectric layers <b>72</b> included in laminated stator core <b>66</b>. As indicated in <figref idref="f0007">FIG. 7</figref>, inner sidewall <b>22</b> of laminated stator core <b>66</b> can be machined to a desired dimension and then an additional dielectric coating <b>76</b> may be applied thereover. In this case, dielectric coating <b>76</b> can contain an inorganic dielectric material having a softening point less than the softening point of the inorganic dielectric particles included in the coating precursor material. In this manner, laminated stator core <b>66</b> can be heated to a temperature less than the softening point of inorganic dielectric particles when forming dielectric coating <b>76</b> to prevent undesired shifting of laminates <b>14</b>. Similarly, outer sidewalls <b>24</b> can be machined to a desired dimension and then an additional dielectric coating <b>74</b> can be applied thereover. This may be unnecessary, however, in embodiments wherein the laminated stator core is subsequently disposed in a casing or can such that outer sidewalls <b>24</b> are pressed against the inner surfaces of the can or otherwise shielded from oxygen exposure.

0040After production of the laminated stator core, additional steps can be performed to integrate the high temperature laminated stator core into an electromagnetic device, such as a motor, sensor, actuator, generator, or magnetic bearing. These additional steps can include the insertion of high temperature prefabricated electromagnetic coils over the stator posts; installation of the stator assembly in a housing using, for example, a shrink fit technique; potting the interior of the housing with an inorganic dielectric filler material; sealing the housing by, for example, welding a lid thereon; and positioning a rotor in the central opening of the stator assembly. Further description of such steps, high temperature electromagnetic coils advantageously combined with the laminated stator cores in manufacturing high temperature electromagnetic devices, and general discussion of high temperature electromagnetic devices can be found in the '476 patent application.

0041The foregoing has thus provided exemplary embodiments of laminated stator cores and methods for producing laminated stator cores capable of providing prolonged and reliable operation at highly elevated temperatures (e.g., temperatures >260°C) at which organic materials tend to breakdown and decompose. In one embodiment, the method includes the steps or processes of depositing a glass-containing coating precursor layer onto one side (the coated side) of an alloy sheet or panel and cutting (e.g., photoetching) a plurality of coated laminates from the alloy panel from the opposing, uncoated side thereof. Prior to deposition of the glass-containing coating precursor layer, an oxidation barrier layer may be formed over the panel utilizing, for example, a thermal growth process or metal plating. Pre-firing is performed after deposition of the glass-containing coating precursor layer to decompose substantially all organic binder and other organic materials therefrom. Pre-firing entails heating the coated laminates to a sintering temperature equal to or less than the softening point of the inorganic dielectric (e.g., glass) particles. The coated laminates are then laid-up in a stack and consolidated into the laminated stator core using heat and convergent pressures. In this manner, inorganic dielectric materials, such as a low melt glass, can be utilized to both electrically insulate and bond the laminates into a rigid stator assembly. The end result is a laminated stator core substantially devoid of organic materials and suitable for usage within high temperature applications at which conventional laminated stator cores rapidly fail. Embodiments of the laminated stator cores are consequently well-suited for integration into efficient electromagnetic devices utilized within high temperature environments, such as high temperature motors, generators, sensors (e.g., RVDTs and motor resolvers), actuators, and magnetic bearings.

0042While primarily described above in the context of a method for manufacturing a high temperature laminated stator core, it will be appreciated that the foregoing has also provided embodiments of a laminated stator core. In one embodiment, the high temperature laminated stator core includes a plurality of laminates each composed of a magnetically-permeable material, such as Fe-Co alloy. A plurality of interlaminate dielectric layers is interspersed or interposed with the plurality of laminates in an alternating relationship. The plurality of interlaminate dielectric layers electrically insulates and bonds together the plurality of laminates. Additionally, the plurality of interlaminate dielectric layers are composed of consolidated glass particles having a softening point less than the melt point of the magnetically-permeable material and further having CTE less than the CTE of the magnetically-permeable material. The plurality of interlaminate dielectric layers are produced from a coating precursor material containing glass particles and one or more organic materials, such as an organic binder. In such cases, substantially all of the organic materials is decomposed from the interlaminate dielectric layers such that the dielectric layers are substantially devoid of organic material; that is, contain less than 1 wt% organic material. In further embodiments, the laminated stator core may also include a plurality of oxidation barrier layers interspersed with the plurality of laminates and the plurality of interlaminate dielectric layers. The plurality of oxidation barrier layers may be selected from the group consisting of a plurality of nickel-plated layers and a plurality of thermally-grown oxide layers.

0043While multiple exemplary embodiments have been presented in the foregoing Detailed Description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing Detailed Description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set-forth in the appended Claims.